Vertical Van Der Waals Heterostructures Research Articles

Charge Transfer in Graphene-MoS2 Vertical Heterostructures Tuned by Stacking Order and Substrate-Introduced Electric Field.

Vertical van der Waals heterostructures composed of graphene (Gr) and transition metal dichalcogenides (TMDs) have created a fascinating platform for exploring optical and electronic properties in the two-dimensional limit. Numerous studies have focused on Gr/TMDs heterostructures to elucidate the underlying mechanisms of charge-energy transfer, quasiparticle formation, and relaxation following optical excitation. Nevertheless, a comprehensive understanding of interfacial charge separation and subsequent dynamics in graphene-based heterostructures remains elusive. Here, we have investigated the carrier dynamics of Gr-MoS2 heterostructures (including Gr/MoS2 and MoS2/Gr stacking sequences) grown on a fused silica substrate under varying photoexcitation energies by comprehensive ultrafast means, including time-resolved terahertz (THz) spectroscopy, THz emission spectroscopy, and transient absorption spectroscopy. Our findings highlight the impact of the substrate electric field on the efficiency of modulating the interfacial charge transfer (CT). Specifically, the optical excitation in Gr/MoS2 generates thermal electron injection from the graphene layer into the MoS2 layer with photon energy well below A-exciton of MoS2, whereas the interfacial CT in the MoS2/Gr is blocked by the electric field of the substrate. In turn, photoexcitation of the A exciton above leads to hole transfer from MoS2 to graphene, which occurs for both Gr-MoS2 heterostructures with opposite stacking orders, resulting in the opposite orientations of the interfacial photocurrent, as directly demonstrated by the out-of-phase THz emission. Moreover, we demonstrate that the recombination time of interfacial exciton is approximately ∼18 ps, whereas the defect-assisted interfacial recombination occurs on a time scale of ∼ns. This study provides valuable insights into the interplay between interfacial CT, substrate effects, and defect engineering in Gr-TMDs heterostructures, thereby facilitating the development of next-generation optoelectronic devices.

Controlled Preparation of High Quality Bubble-Free and Uniform Conducting Interfaces of Vertical van der Waals Heterostructures of Arrays.

Sharp and clean interfaces of van der Waals (vdW) heterostructures are highly demanded in two-dimensional (2D) materials-based devices. However, current assembly methods usually cause interfacial bubbles and wrinkles, hindering carrier interlayer transport. The preparation of a large-scale vdW heterostructure with a bubble-free interface is still a challenge. Although many efforts have been made to eliminate bubbles, the evolution processes of the interfacial bubbles are rarely studied. Here, the interface bubble formation and evolution of the transferred 2D materials and their vdW heterostructure are systemically studied by the atomic force microscopy (AFM) technique and high-resolution surface current mapping. A thermal annealing procedure is developed to reduce the number of bubbles and to improve the quality of interfaces. In addition, influences of the interface residues and nanosteps on bubble evolution are also discussed. Further, we develop the polystyrene (PS)-mediated polydimethylsiloxane (PDMS) transfer technique to realize the high-quality transfer of heterostructure arrays. Finally, high-resolution surface current mapping results confirm that we can now produce highly uniform electrical conduction interfaces of heterojunctions. This study provides guidance for assembling high quality interfaces and paves the way for production of bubble-free heterostructure-based electronic devices with high performance and good uniformity.

Tailored Synthesis of Heterogenous 2D TMDs and Their Spectroscopic Characterization

Two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) show great potential across various applications. However, synthesizing large-scale structures poses challenges owing to the intricate growth parameters, forming unexpected hybrid film structures. Thus, precision in synthesis and thorough structural analysis are essential aspects. In this study, we successfully synthesized large-scale structured 2D transition metal dichalcogenides (TMDs) via chemical vapor deposition using metal oxide (WO3 and MoO3) thin films and a diluted H2S precursor, individual MoS2, WS2 films and various MoS2/WS2 hybrid films (Type I: MoxW1−xS2 alloy; Type II: MoS2/WS2 vdWH; Type III: MoS2 dots/WS2). Structural analyses, including optical microscopy, Raman spectroscopy, transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy, and cross-sectional imaging revealed that the A1g and E2g modes of WS2 and MoS2 were sensitive to structural variations, enabling hybrid structure differentiation. Type II showed minimal changes in the MoS2′s A1g mode, while Types I and III exhibited a ~2.8 cm−1 blue shift. Furthermore, the A1g mode of WS2 in Type I displayed a 1.4 cm−1 red shift. These variations agreed with the TEM-observed microstructural features, demonstrating strain effects on the MoS2–WS2 interfaces. Our study provides insights into the structural features of diverse hybrid TMD materials, facilitating their differentiation through Raman spectroscopy.

Tunable long-range spin transport in a van der Waals Fe3GeTe2/WSe2/Fe3GeTe2 spin valve.

The seamless integration of two-dimensional (2D) ferromagnetic materials with similar or dissimilar materials can widen the scope of low-power spintronics. In this regard, a vertical van der Waals (vdW) heterostructure of 2D ferromagnets with semiconducting transition metal dichalcogenides (TMDCs) forms magnetic junctions with exceptional stability and electrical control. Interestingly, 2D metallic Fe3GeTe2 (FGT) reveals above room temperature Curie temperatures and has large magneto anisotropy due to spin-orbit coupling. In addition, it also possesses topological states and a large Berry curvature. Herein, we designed the FGT/WSe2/FGT vdW heterostructure with a uniform and sharp interface so that FGT could maintain its inherent electronic properties. Also, the uniform thickness of the barrier provides a smooth flow of spins through the junctions as tunneling exponentially decays with an increasing barrier thickness. However, strong energy-dependent spin polarization is crucial for achieving optimum spin valve properties, such as large tunneling magnetoresistance (TMR) along with the manipulation of the magnitude and sign reversal. We have observed a shifting of high-energy localized minority spin states toward low-energy regions, which causes spin polarization fluctuation between -42.5% and 41% over a wide range of bias voltage. This leads to a negative TMR% of ∼-100% at 0.1 V Å-1 and also a large positive TMR% at 0.2 V Å-1 and -0.4 V Å-1. Besides, the system exhibits a highly tunable large anomalous Hall conductivity (AHC) of 626 S cm-1. Interestingly, such unprecedented electronic behaviour with large and switchable spin polarization, anomalous Hall conductivity and TMR can be incorporated into MTJ devices, which provide electrical control and long-range spin transport. Additionally, the system emerges as a standout candidate in low-power spintronic devices (e.g., MRAM and magnetic sensors) owing to its distinctive energy-dependent electronic structure with a wide range of external bias.

(Invited) Interlayer Excitons in Two-Dimensional Perovskite/Monolayer Transition Metal Dichalcogenide Heterostructures

The emergence of two-dimensional (2D) materials has intrigued a great deal of research on novel physical phenomena and various functional applications due to their particular crystal structures and reduced dimensionality. Unlike 3D materials, bulks of 2D materials can be easily thinned to atomic thickness by mechanical exfoliation and atomically thin 2D materials can be arbitrarily stacked to form vertical van der Waals (vdW) heterostructures, which could inherit the unique characteristics of the constituent layers and even exhibit new properties not possessed by them. Particularly, when type-Ⅱ vdW heterostructures are excited, the positive and the negative charges would reside in the different layers after the charge transfer but be limited within a short distance due to the strong quantum confinement effect of 2D materials, leading to strong Coulomb interaction and the formation of interlayer excitons (IXs). IXs are generally equipped with orientated dipole moment and long lifetime, making them ideal media for the future interconnects between optical transmission and electronic computation. Currently, studies on IXs are mainly focused on the vdW heterostructures formed by monolayers of transition-metal dichalcogenides (TMDs). Here, I will first talk about vdW heterostructures formed by 2D perovskites and TMDs for studying IXs. Stacking different kinds of 2D perovskites and TMDs, formation of IXs is confirmed by excitation-power-, temperature-, electric-field-dependent and time-resolved photoluminescence (PL) studies. Notably, robust IX emission can be observed regardless of the stacking sequence and geometric alignment of the constituent layers, showing great advantages over the TMD/TMD vdW heterostructures, which require special twist-angle and thermal annealing. Then, I would like to give a brief introduction on widely tuning the IX emission energy by changing the layer number of the 2D perovskite or the TMD, which shed light on the application of 2D perovskite/TMD vdW heterostructures in broad-spectrum optoelectronics. Furthermore, I would like to next talk about how the selection of organic chains in 2D perovskites influences the properties of the IXs. By using chiral 2D perovskites, the IX emission shows substantial circular polarization and the polarization direction is only related to the chirality of the molecules regardless of the excitation scheme or any other external field, which could open up new passages for controlling valley- or spin-polarization of IXs. By introducing molecules with different dielectric constants, the dielectric screening strength in the vdW heterostructure is changed and hence the binding energy of the IXs is also modified, which offers great opportunities for exploiting tunable excitonic devices and studying exciton condensation. Finally, I will also introduce IX as a non-destructive tool for probing the local phase transition at the surface of the 2D perovskite (BA)2PbI4 flakes. By spatially PL mapping of the (BA)2PbI4/WSe2 heterostructure, two different IX species can be observed to distinguish the low-temperature and the high-temperature phase respectively.

Van der Waals heterostructure of Bi2O2Se/MoTe2 for high-performance multifunctional devices

Two-dimensional van der Waals heterostructures exhibit distinctive electronic and optoelectronic properties, making them promising structures for constructing advanced multifunctional devices. However, devices based on conventional charge-carrier transport mechanisms often perform only a single function, which limits its integration and performance. Here, we present a vertical van der Waals heterostructure made of Bi2O2Se and MoTe2, allowing it to act as high-performance backward diode, forward diode, photodetector and photovoltaic device at various working conditions. The applications are enabled by band-alignment switching between p–n heterostructure controlled by minority carrier diffusion and n–n heterostructure governed by the thermionic emission and tunneling-mediated processes. As a backward diode, the device displays a high reverse rectification ratio of 5.0 × 104. As a photodetector, the device demonstrates a broad spectral photoresponse ranging from ultraviolet (365 nm) to near-infrared (1050 nm). When irradiated by 532 nm laser, the photodetector shows a responsivity of up to 11.6 A/W and achieves quick response/recovery speed of 19.6/8.8 μs. As a photovoltaics device, an external quantum efficiency of 78% and a responsivity of 0.33 A/W are observed. This study showcases the potential for high-performance multifunctional devices utilizing Bi2O2Se/MoTe2 heterostructures and provides comprehensive insights into the designed band alignment and its applications.

High-Performance Infrared Detectors Based on Black Phosphorus/Carbon Nanotube Heterojunctions.

Infrared detectors have broad application prospects in the fields of detection and communication. Using ideal materials and good device structure is crucial for achieving high-performance infrared detectors. Here, we utilized black phosphorus (BP) and single-walled carbon nanotube (SWCNT) films to construct a vertical van der Waals heterostructure, resulting in high-performance photovoltaic infrared detectors. In the device, a strong built-in electric field was formed in the heterojunction with a favored energy-band matching between the BP and the SWCNT, which caused a good photovoltaic effect. The fabricated devices exhibited a diode-like rectification behavior in the dark, which had a high rectification ratio up to a magnitude of 104 and a low ideal factor of 1.4. Under 1550 nm wavelength illumination, the 2D BP/SWCNT film photodetector demonstrated an open-circuit voltage of 0.34 V, a large external power conversion efficiency (η) of 7.5% and a high specific detectivity (D*) of 3.1 × 109 Jones. This external η was the highest among those for the photovoltaic devices fabricated with the SWCNTs or the heterostructures based on 2D materials and the obtained D* was also higher than those for most of the infrared detectors based on 2D materials or carbon materials. This work showcases the application potential of BP and SWCNTs in the detection field.

Realization of high thermoelectric performance of black phosphorus / black arsenic hybrid heterojunction nanoscale devices by interface engineering

Thermoelectric properties of hybrid lateral heterostructure (LHS) and vertical van der Waals heterostructure (vdWHS) nanodevices of black phosphorus and black arsenic are investigated based on first-principle combined with non-equilibrium Green's function and density functional theory. Results show that thermoelectric properties are significantly enhanced by constructing vertical van der Waals heterostructures, and high ZT values are derived from energy filtering effect of the interface for low and medium frequency phonons when a weak coupling contact is adopted. Due to the decrease of range of effective phonon transmission and spectrum, the vertical van der Waals heterostructure with a smaller contact area and slightly lower phonon thermal conductance finally shows better thermoelectric performance and optimal ZT value is close to 3.5 when T is 350 K. Our theoretical work provides a new approach and theoretical guidance for achieving high performance of high-density integrated circuits in microelectronics industry.

Enhanced in-plane thermoelectric properties achieved through the vertical van der Waals stacking of black phosphorus and Ti2C

Vertical van der Waals heterostructures (vdWHs) exhibit considerable freedom in integrating two-dimensional layered materials for achieving excellent performance. In this study, we construct a double-layer Ti2C/BP vdWH by stacking pristine monolayer black phosphorus (BP) and Ti2C vertically. The in-plane thermoelectric properties of BP, Ti2C and Ti2C/BP vdWHs at different temperatures are investigated using density functional theory combined with the non-equilibrium Green's function method. Results showed that the thermoelectric figure of merit (ZT) of Ti2C/BP vdWHs at room temperature (300 K) was approximately 103 and 104 times that of Ti2C and BP, respectively. Because of the mutual compensation of the electronic energy bands and interfacial phonon scattering, Ti2C/BP vdWHs have higher electron conductance and lower phonon conductance than both BP and Ti2C, which significantly enhances the ZT value of these structures. More importantly, the prominent thermoelectric properties of Ti2C/BP vdWHs maintain greater stability as the temperature rises (300–900 K), which leads to a high thermoelectric power factor and ZT value without relying on high temperatures. These results provide a new idea for designing two-dimensional functionalised nanoscale devices with excellent thermoelectric performance.

High Carrier Mobility and Controllable Electronic Property of the h-BN/SnSe2 Heterostructure.

Building two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) is one of the effective methods to regulate the properties of single 2D materials. In this paper, we stack the hexagonal boron nitride (h-BN) monolayer (ML) on the SnSe2 ML to construct the stable h-BN/SnSe2 vdWH, of which the crystal and electronic structures, together with the optical properties, are also analyzed by the first-principles calculations. The results show that the h-BN/SnSe2 vdWH belongs to a type-I heterostructure with an indirect bandgap of 1.33 eV, in which the valence band maximum and conduction band minimum are both determined by the component SnSe2 ML. Interestingly, the h-BN/SnSe2 vdWH under the tensile strain or electric field undergoes the transitions both from type-I to type-II heterostructure and from the indirect to direct bandgap semiconductor. In addition, the carrier mobility of the h-BN/SnSe2 heterostructure has a significant enhancement relative to that of the SnSe2 ML, up to 104 cm2 V-1 s-1. Meanwhile, the h-BN/SnSe2 heterostructure presents the superb optical absorption and unique type-II hyperbolic property. Our findings will broaden the potential applications of SnSe2 ML and provide theoretical guidance for the related experimental studies.

Lattice Reconstruction in MoSe2–WSe2 Heterobilayers Synthesized by Chemical Vapor Deposition

Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moiré systems with rich correlated electron phases and moiré exciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe2-WSe2, however, lattice reconstruction eliminates the canonical moiré pattern and instead gives rise to arrays of periodically reconstructed nanoscale domains and mesoscopically extended areas of one atomic registry. Here, we elucidate the role of atomic reconstruction in MoSe2-WSe2 heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods, we identify the coexistence of moiré-type cores and extended moiré-free regions in heterostacks with parallel and antiparallel alignment. Our work highlights the potential of chemical vapor deposition for applications requiring laterally extended heterosystems of one atomic registry or exciton-confining heterostack arrays.

Giant tunnel electroresistance in two-dimensional ferroelectric tunnel junctions constructed with a Sc2CO2/In2Se3 van der Waals ferroelectric heterostructure

Two-dimensional (2D) ferroelectric materials have attracted great attention in recent years due to their thin thickness, high stability, and switchable polarization states. In particular, ferroelectric tunnel junctions (FTJs) constructed from 2D ferroelectric materials have been shown to have very high tunnel electroresistance (TER) ratios. In this work, we design a ferroelectric tunnel junction composed of ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}/{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ vertical van der Waals (vdW) heterostructure based on two different 2D ferroelectric materials with out-of-plane polarization. Through density functional calculations combined with a nonequilibrium Green's function technique, it is found that the TER ratio as high as ${10}^{7}%$ can be achieved. Analysis shows that it originates from the difference in the work functions of the contact surfaces of the two ferroelectric materials, which makes charge transfer occur or not occur between them and further leads to a metalinsulator switching of the ferroelectric vdW heterostructure upon the reversion of the applied electrical field. The results suggest the importance of ferroelectric vdW heterostructure in the design of FTJs and a feasible design scheme characterized by proper choice of the work functions and band gaps of the component materials.

Record-High Work-Function p-Type CuBiP2 Se6 Atomic Layers for High-Photoresponse van der Waals Vertical Heterostructure Phototransistor.

The notable lack of intrinsic p-type 2D layered semiconductors has hindered the engineering of 2D devices for complementary metal oxide semiconductors (CMOSs). Herein, a novel quaternary intrinsic p-type 2D semiconductor, CuBiP2 Se6 atomic layers, is introduced intothe2D family. The semiconductor displays a high work function of 5.26eV, a moderate hole mobility of 1.72 cm2 V-1 s-1 , and an ultrahigh on/off current exceeding 106 at room temperature. To date, 5.26eV is the highest work-function recorded in p-type 2D materials, indicating the ultrastable p-type behavior of CuBiP2 Se6 . Additionally, a multilayer graphene/CuBiP2 Se6 /multilayer graphene (MLG/CBPS/MLG)-based fully vertical van der Waals heterostructure phototransistor is designed and fabricated. This device exhibits outstanding optoelectronic performance with a responsivity (R) of 4.9 ×104 A W-1 , an external quantum efficiency (EQE) of 1.5 ×107 %, a detectivity (D) of 1.14 ×1013 Jones, and a broad working wavelength (400-1100nm), respectively. This is comparableto state-of-the-art 2D devices. Such excellent performance is attributed to the ultrashort transmit length and nondestructive/defect-free contacts. This leads to faster response speed and eliminates Fermi-levelpinningeffects. Moreover, ultrahigh responsivity and detectivity endow the device with applaudable imaging sensing capability. These results make CuBiP2 Se6 an ideal p-type candidate material for next-generation CMOSs logic devices.

Optical modeling of single and multilayer two-dimensional materials and heterostructures

Bidimensional materials are ideally viewed as having no thickness, as their name suggests. Their optical response have been previously modelled by a purely bidimensional surface current or by a very thin film with some contradictory results. The advent of multilayer stacks of bidimensional materials and combinations of different materials in vertical van der Waals heterostructures highlights, however, that these materials have a finite thickness. In this article, we propose a new model that reconciles both approaches and we show how volume properties of stacked bidimensional layers can be calculated from the bidimensional response of each individual layer, and conversely. In our approach, each bidimensional layers is surrounded by vacuum and described as a kind of transfer matrix with intrinsic parameters that do not depend on the external medium. This provides a link between continuous thin films and discrete layers. We show how to model heterostructures of bidimensional materials and identify the parameters of the current sheet that represents the bidimensional material in the zero-thickness limit, namely the in-plane surface susceptibility and the out-of-plane displacement susceptibility. We show that our unified model is perfectly compatible with existing ellipsometric data with the same reliability as the existing interface model but with different values of the surface susceptibility or bulk dielectric function. We discuss in detail the origin of the discrepancies and show that our approach allows to determine intrinsic properties of the bidimensional materials with the advantage that multilayer and monolayer systems are described in a same framework.

MoTe2/InN van der Waals heterostructures for gas sensors: a DFT study.

Vertical van der Waals (vdW) heterostructures have shown potential for gas sensing owing to their remarkable sensitivity. However, the optimization process for achieving the best gas sensing performance is complicated by the heterostructure's reliance on both physical and electrical characteristics. This study employs density functional theory (DFT) to analyse the structural and electronic parameters of a MoTe2/InN vdW heterostructure. The findings of this study indicate that the vdW heterostructure has a type-II band alignment with higher adsorption energy towards NH3, NO2, and SO2 than the individual monolayers. In specific, the heterostructure is well suited for NO2 detection but has limitations in reliably detecting NH3 and SO2 due to longer recovery times. We find significant hybridization between the adsorbate and interacting surfaces' orbitals and a notable presence of NO2 molecular orbitals in proximity to the Fermi level. Additionally, dielectric and work function modulations offer a viable means to develop optical-based gas sensors that can selectively detect NO2. Our research provides valuable insights into vdW heterostructure design for high-performance gas sensors.

Complex nature of the interaction of AFM tip with the freshly splitted (010) surface of CdWO4 and ZnWO4 single crystals

The discovery of new low-dimensional materials formed by splitting known bulk crystals into separate elementary layers has enabled future realization of still undiscovered potential of long-studied materials. High-resistivity two-dimensional films obtained by splitting ionic layered crystals are of great interest for creating new vertical van der Waals heterostructures. For unveiling the potential of using two-dimensional monocrystalline films of CdWO4 and ZnWO4, it is necessary to gain a more penetrating insight into the processes that occur on the freshly splitted surface of the crystals. In the present study, for the first time we investigated the effect of AFM probe on the (010) surfaces of freshly splitted CdWO4 and ZnWO4 single crystals. It is shown that the observed “protruding areas” on the splitted surfaces of these crystals are of a complex nature, and they are mainly related with the transfer of charges, as well as with the formation of complete bonds on the surface of the broken crystal lattice of examined crystals. The charge spots observed on the splitted surface of the crystals arise as a result of local charging by AFM probe. As a result of charge diffusion, as well as under the influence of ambient medium, the complete dissipation of charges occurs in less than 2 h.

Van der Waals epitaxial growth and optoelectronics of a vertical MoS2/WSe2 p–n junction

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attention due to their unique electronic and optical properties. In particular, TMDs can be flexibly combined to form diverse vertical van der Waals (vdWs) heterostructures without the limitation of lattice matching, which creates vast opportunities for fundamental investigation of novel optoelectronic applications. Here, we report an atomically thin vertical p–n junction WSe2/MoS2 produced by a chemical vapor deposition method. Transmission electron microscopy and steady-state photoluminescence experiments reveal its high quality and excellent optical properties. Back gate field effect transistor (FET) constructed using this p–n junction exhibits bipolar behaviors and a mobility of 9 cm2/(V·s). In addition, the photodetector based on MoS2/WSe2 heterostructures displays outstanding optoelectronic properties (R = 8 A/W, D* = 2.93 × 1011 Jones, on/off ratio of 104), which benefited from the built-in electric field across the interface. The direct growth of TMDs p–n vertical heterostructures may offer a novel platform for future optoelectronic applications.Graphical

Vertical MoSe2–WSe2 p-n Heterojunctions Grown by One-Pot Chemical Vapor Deposition for Ultrafast Photodetection

Two-dimensional transition metal dichalcogenides (TMDCs) have been extensively studied in the last decade due to their atomic thin structure and unique optoelectronic properties. In particular, lateral and vertical van der Waals heterostructures by combining different transition metal dichalcogenides (TMDCs) have been studied to develop the potential applications of two-dimensional materials. However, the optical properties are challenging due to the limited quality of TMDCs based heterojunctions. In this work, vertical MoSe2/WSe2 p–n heterojunctions have been synthesized by one-pot chemical vapor deposition (CVD) method. The MoSe2 -WSe2 heterostructure exhibits rectification characteristics of a p-n junction with the rectification ratio of 100 at ±10V bias. The phototransistor based on MoSe2/WSe2 p-n heterojunction shows a photoresponse of visible(638nm). The responsivity can reach up to 20 A/W, and the detectivity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.5\times 10^{13}$ </tex-math></inline-formula> Jones. Most importantly, the optical response time of the MoSe2/WSe2 p-n heterojunction is as fast as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$28 ~\mu $ </tex-math></inline-formula> s. The results indicate that vertical MoSe2/WSe2 p-n heterojunction could be a prospective candidate for high-performance optoelectronic devices.

Sequential Growth of Vertical Transition-Metal Dichalcogenide Heterostructures on Rollable Aluminum Foil.

Vertical van der Waals heterostructures (vdWhs), which are made by layer-by-layer stacking of two-dimensional (2D) materials, offer great opportunities for the development of extraordinary physics and devices such as topological superconductivity, robust quantum Hall phenomenon, electron-hole pair condensation, Coulomb drag, and tunneling devices. However, the size of vdWhs is still limited to the order of a few micrometers, which restricts the large-scale roll-to-roll processes for industrial applications. Herein, we report the sequential growth of a 14 in. vertical vdWhs on a rollable Al foil via chemical vapor deposition. By supplying chalcogen precursors to liquid transition-metal precursor-coated Al foils, we grew a wide range of individual 2D transition-metal dichalcogenide (TMD) films, including MoS2, VS2, ReS2, WS2, SnS2, WSe2, and vanadium-doped MoS2. Additionally, by repeating the growth process, we successfully achieved the layer-by-layer growth of ReS2/MoS2 and SnS2/ReS2/MoS2 vdWhs. The chemically inert Al native oxide layer inhibits the diffusion of chalcogen and metal atoms into Al foils, allowing for the growth of diverse TMDs and their vdWhs. The conductive Al substrate enables the effective use of vdWhs/Al as a hydrogen evolution reaction electrocatalyst with a transfer-free process. This work provides a robust route for the commercialization of 2D TMDs and their vdWhs at a low cost.

Electrically Controlled Wavelength-Tunable Photoluminescence from van der Waals Heterostructures.

Achieving facile control of the wavelength of light emitters is of great significance for many key applications in optoelectronics and photonics, including on-chip interconnection, super-resolution imaging, and optical communication. The Joule heating effect caused by electric current is widely applied in modulating the refractive index of silicon-based waveguides for reconfigurable nanophotonic circuits. Here, by utilizing localized Joule heating in the biased graphene device, we demonstrate electrically controlled wavelength-tunable photoluminescence (PL) from vertical van der Waals heterostructures combined by graphene and two-dimensional transition metal dichalcogenides (2D-TMDCs). By applying a moderate electric field of 6.5 kV·cm-1 to the graphene substrate, the PL wavelength of 2D-TMDCs exhibits a continuous tuning from 662 to 690 nm, corresponding to a bandgap reduction of 76 meV. The electric control is highly reversible during sweeping the bias back and forth. The temperature dependence of Raman and PL spectroscopy reveals that the current-induced local Joule heating effect plays a leading role in reducing the optical direct bandgap of TMDCs. The bias-dependent optical reflectivity and time-resolved photoluminescence measurements show a consistent reduction of the optical band gap of 2D-TMDCs and increased PL lifetimes with the electric field over the heterostructures. Moreover, we demonstrate the consistent device operation from 2D materials grown by chemical vapor deposition, showing great advantages for the scalability.